Electronic device with a programmable resistive element and a method for blocking a device

ABSTRACT

One or more embodiments relate to an electronic device comprising a circuitry and a programmable resistive element. The programmable resistive element comprises a first and a second state, wherein the programmable resistive element is configured to allow switching from the second state into the first state in response to a signal comprising at least a predefined level. The circuitry is configured to provide signals up the predefined level, wherein the circuitry is configured to provide a switch signal to the programmable resistive element, wherein the switch signal causes switching from the first into the second state.

BACKGROUND

Embodiments of the present invention relate to an electronic device witha programmable resistive element. Further embodiments relate to anelectronic device with a phase change memory element. Yet otherembodiments relate to an electronic device with an asymmetricphase-change memory cell.

There is a need for a fast programmable non-volatile memory cellcomprising an irreversible state, wherein the one or more irreversiblestates may physically be irreversible, and a programming speed should beespecially high.

SUMMARY

Embodiments of the present invention relate to an electronic devicecomprising a circuitry and a programmable resistive element. Theprogrammable resistive element comprises at least a first and a secondstate, wherein the programmable resistive element is configured to allowswitching from the second state into the first state in response to asignal comprising at least a predefined level. The circuitry isconfigured to provide signals below the predefined level, wherein thecircuitry is configured to provide a switch signal to the programmableresistive element, wherein the switch signal causes switching from thefirst into the second state.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be explained in the followingwith reference to the accompanying drawings, in which:

FIG. 1 shows a schematic view of the electronic device according to anembodiment of the present invention;

FIGS. 2 a and 2 b show a cross-sectional and an overview of anasymmetric phase-change memory cell;

FIGS. 3 a and 3 b show the asymmetric phase-change memory cell withshifted amorphous regions;

FIG. 4 shows a dependence of the resistance on a reset current; and

FIG. 5 shows an MOS-device comprising the asymmetric phase-change memorycell.

Before embodiments of the present invention are explained in greaterdetail on the basis of the drawings in the following, it is to bepointed out that like elements in the figures are provided with the sameor similar reference numerals, and that a repeated description of theseelements is omitted.

DETAILED DESCRIPTION OF THE INVENTION

Programmable cells can be programmed to switch between different statesrelated to information stored by the cells (e.g. a high ohmic and a lowohmic state). The states can be reversible or irreversible such that thecell can switch between reversible states or into the irreversiblestate, but there should no return possible from an irreversible (ornon-reversible) state. One simple possibility to form an irreversiblestate is to set a flag, which blocks operations as, for example, themodification of the cell content.

Conventional cells with at least one irreversible state comprising saidone time programming device or the logical obstruction, which blocks arewriting of the device by setting a flag in a non-volatile memory.

Conventional one time programming (OTP) device may rely on differentphysical effects. Examples are fuses or electrically erasableprogrammable read-only memory (EEPROM) or flash memory cells, which areconfigured so that their content cannot be removed or erased oroverwritten. The OTP device comprises two states, wherein in most casesone of the two states is irreversible, and the OTP cells are accordingto their name only programmable once. The flag of the OTP device mayindicate that a specification of the individual integrated circuit (IC)is invalid. This specification is however still present implying that apotential attacker could in principle by circumventing this flag, againaccessing the specification (for example by a physical bridging of theflag). An additional drawback of these known methods is the insufficientspeed or time period, which is needed to program the OTP devices as wellas the high-voltage needed for programming of fuses.

On the other hand, in case of a logical blocking against overwriting(locking) it is possible to program the cell multiple times beforeswitching the cell into a locked state. This provides, however, only alogical protection, so that the security is, in general, not as high asfor a physical protection. The locked state is moreoverindistinguishable from the unlocked state or, in other words, additionalinformation is required to distinguish between both states.

Embodiments of the present invention aim to provide a non-volatile fastprogrammable cell with irreversible states, which are—without physicalaction from outside—physically non-reversible. Hence, the irreversiblestate is not only blocked by a logical obstruction (for example bysetting a specific flag) for changing the states. On the other hand,embodiments allow switching the cell between the two reversible stateswith almost no limitations with respect to the number of changes as longas the cell is not switched into the irreversible state or into one ofits irreversible states.

Embodiments relate therefore to an electronic device comprising acircuitry and an asymmetric phase change memory cell comprising at leasta first, a second and a third state. The asymmetric phase change memorycell is configured to allow switching from the third state into thefirst or into the second state in response to a signal comprising atleast a predefined level. The circuitry is configured to provide a firstand a second signal to the asymmetric phase change memory cell, whereinthe first signal causes switching between the first and the second stateand the second signal causes switching from the first or from the secondstate into the third state. The circuitry is configured to providesignals up to a maximum level, which is below the predefined level.

In contrast to conventional one time programming cells, embodimentsdefine a cell that may physically be changed so that the cell may not beprogrammable anymore. The irreversible state can therefore bedistinguished from the other two reversible states, which is not thecase for conventional logical obstructions (for performing certainoperations).

Another set of problems deals with the protection of the generation of adevice specific bitstream K_(I) (=key), which can be generated by anintegrated circuit. Such circuits can, for example, be realized in termsof a so-called physically uncloneable function (PUF). A PUF circuit isemployed, for example, in identification devices used to implement asecret key mutual authentication protocol. Said identification devicesas, for example, RFID chips (RFID=Radio Frequency Identification) may beused for identifying the owner of the chip or the product to which thechip is connected. The physical uncloneable function comprises, forexample, a circuit, which exploits chip-to-chip process variations andgenerates on the fly a volatile chip individual key.

The PUF offers a unique and unpredictable way of mapping challenges toresponses (=the uncloneable function), although different PUFs may havebeen manufactured by the same process and may comprise the same devices.The uncloneability property is related to unavoidable manufacturingvariations so that each device differs from another device in theirelectric characteristics. In silicon PUFs for example, the switchingdelays of transistors may deviate from each other, so that a simple timemeasurement for a signal propagating along different paths may differfrom PUF to PUF in an unpredictable way. The manufacturing tolerancesas, for example, layer thickness, doping levels, geometrical distances,etc. are namely controllable only up to certain accuracy. For example,there are no two transistors, whose threshold voltages are exactly thesame. Due to this unpredictability, every PUF comprises its ownchallenge-response authentication.

A PUF-generated secret key guarantee therefore a higher security againstcloning than a key programmed in a non-volatile memory because tamperingattempts will destroy the key.

Therefore, the PUF comprises, for example, the following properties:

(a) the uncloneability, implying that a potential attacker is unable togenerate the same bitstream K_(I) with an identical circuit, nor is heable to predict the integrated circuit-specific bitstream from hisknowledge of the circuit;

(b) the uniqueness, meaning that the same circuit generates on eachintegrated circuit a different and, hence, device individual bitstream.

Therefore, an indistinguishable and uncloneable fingerprint is generatedby the integrated circuit (in the broadest sense).

Embodiments of the present invention are also configured to destroy thephysically uncloneable function irreversibly and quickly, respectivelythe device-specific key K_(I). This operation ultimately terminates thelifecycle of the integrated circuit and, therewith destroys the accesskeys, ID-cards, passports, etc. To achieve this, embodiments use anasymmetric phase-change memory cell or programmable resistive elementscomprising at least one irreversible state, so that the device isswitched into the irreversible state in case an attack is detected, orin the case of a predetermined lifetime of an integrated circuit havingexpired.

Therefore, embodiments comprise also a security device comprising anasymmetric phase change memory cell (or in general a programmableresistive element) comprising at least a first, a second and a thirdstate, wherein the asymmetric phase change memory cell is configured toallow switching from the third state into the first or into the secondstate in response to a signal comprising a predefined level. Thesecurity device comprises moreover a circuitry configured to provide adevice individual bit stream and to proved a first and a second signalto the asymmetric phase change memory cell, wherein the first signalcauses switching between the first and the second state and the secondsignal causes switching from the first or from the second state into thethird state. The circuitry is further configured to provide signals upto a maximum level, which is below the predefined level, and wherein thecircuitry is configured to block providing the bit stream in case theasymmetric phase change memory cell is within the third state.

Phase change memory (PCM or PCRAM) cells comprise a phase changematerial, which can be in a crystalline or amorphous phase.Correspondingly the PCM cell comprises at least two states, one highohmic (amorphous) and one low ohmic (crystalline) state. Alternatively,the PCM cell may comprise two crystalline (low ohmic) and only oneamorphous (high ohmic) state, or two crystalline and two amorphousstates. The change between the states is initiated by different signals.For example, to switch from the amorphous to the crystalline state along signal (set signal) with low current is needed (long enough thatthe atoms can form a crystal structure), whereas the switching from thecrystalline to the amorphous phase is done be short signal (resetsignal) with high current (after heating the cooling occurs quickly sothat no crystal can be formed). Examples for the time period and thecurrents are: for the reset signal: about 100 . . . 800 μA for about 10. . . 50 ns and for the set signal: about 50 . . . 150 μA for about 100. . . 300 ns.

The phase-change memory cells are one of the best performing candidatesfor scaled non-volatile memories, as flash technologies face majorscaling issues. During the programming of the PCRAM, both a highelectrical current density and a steep temperature gradient occur. Dueto the good electrical and poor thermal conductivity of Chalcogenidealloys as phase change material, strong thermo-electric effects arepresent.

For example, the Thomson Effect renders the temperature distributionalong the phase-change material asymmetric (between both electrodes),since only part of the phase change material exhibits a temperature,which is above the melting temperature, and hence, which becomesamorphous by quenching. This amorphous portion is shifted within thephase-change material) by the thermo-electric effect. Due to theasymmetry of the cell, the Thomson-induced shift of the amorphous arearesults in different electrical performance when polarity is reversed.

Embodiments of the present invention comprise an asymmetric PCM memorycell (e.g. a T-Cell). These asymmetric phase-change memory cellscomprise a crystalline state (the first state) and two amorphous states(the second and the third state), which differ in their resistancecaused by the asymmetry with respect to the polarity (see below). Thesecond state may comprise for example a lower resistance than the thirdstate, so that the first and the second state comprise the reversiblestates, whereas the third state (e.g. with the highest resistance of thethree states) comprise the irreversible state. In addition, inembodiments the phase change material of the asymmetric phase-changememory cell may comprise as phase change material, e.g., a chalcogenideor a chalcogenide alloy.

In embodiments the working voltage or the maximal current, which isimplemented on the chip, is chosen such that switching between the firstand second states (reversible states) are possible without limitations,i.e. a sufficient heating for the phase-change from the crystalline tothe amorphous state can be achieved. But on the other hand, the workingvoltage (or current) is too low (or comprise the wrong polarity) toprovide a sufficient heating for the third state (the high Ohmicamorphous state) so that phase change into the crystalline phase isexcluded. In other words, the circuitry is configured such that theneeded signal for this comprises a level will that will damage thecircuitry.

In another embodiment, the electronic device comprises two crystallinestates, wherein one of the crystalline states comprises a low Ohmicstate that is irreversible and cannot be changed (over-written) afterthe device has switched into this state. Again the reason is that thedevice is unable to provide the needed signal for this phase change,since the electronic device is unable to provide a reset current tochange this state.

In yet another embodiment, the electronic device comprises twoirreversible states: one irreversible crystalline state and oneirreversible amorphous state. These two irreversible states may becombined with one or two reversible states, wherein the resistances ofthe reversible states may be in between the resistances of theirreversible states, i.e. the state with the highest and lowestresistivity may comprise the irreversible states.

Therefore, embodiments use PCRAM-cells with at least three states, whichare distinguishable by their electrical resistance, and hence may beread by a measurement of the resistance, which distinguished the threestates. Embodiments comprise moreover a maximum programmable voltage,which is such that a state change out of one of the low Ohmic states ispossible, but from the high Ohmic state (third state), is not.

In embodiments the asymmetric phase change memory needs for example apredefined level for switching from the third state into the first orsecond stat comprises a predefined voltage or a predefined current or apredefined polarity, wherein the circuitry is unable to provide thepredefined voltage or the predefined current or the predefined polarity.

In yet further embodiments the circuitry comprises a detector fordetecting an unauthorized access (attack) of the circuitry or a meansfor processing. The asymmetric phase change memory cell is configured toswitch into the third state in case the detector detects theunauthorized access. The detector can, e.g., send an alert signal to thecircuitry, which controls the switching into the third state.

These PCRAM cells, can for example, be used:

-   -   for management of lifecycles of products such as smartcards;    -   to store an alert state after a successful attack on the        security chip, in which case the high writing speed is        important. Such devices can, for example, comprise security        relevant static information, for instance keys used in        cryptography. In the case of a detected attack, the asymmetric        phase-change cell is put into the irreversible state, whereby        the sensitive information is automatically erased; the attack is        however monitored (e.g. for later inspection);    -   the devices can also be used for lock bits in order to provide a        logical overwrite protection. When using embodiments the lock        bits are, however, physically irreversible.

Embodiments comprise numerous advantages that can be summarized asfollows. If the electronic device comprises a physically uncloneablefunction and the asymmetric phase-change memory cell is used toimplement such PUF (for example in the low Ohmic state), then it ispossible to destroy, quickly and efficiently, the device's individualcharacteristics. This can be achieved by switching the phase-changememory cell into the third, the so-called OTP state. In this case, thePUF is no longer able to generate the IC-individual characteristics(e.g. a key) used, for example, for identification. Since the changeinto the third state is physically not reversible, the individualcharacteristics are ultimately destroyed making, e.g., theidentification device invalid and hence not usable for an unauthorizedperson.

Physically irreversible means that without physical action from outside(which may destroy the circuitry) it is impossible to change this state.Therefore, it is not only a logical obstruction to switch a state.

This physically irreversibility comprises already a major advantage ofthis realization, which is combined with the fast mechanism so that evenunder an attack condition the switching into the third state can stillbe performed. Also the low energy consumption is important to performthe switching into the third state even during the attack (no chargepumping is needed). Hence, the programming of the non-volatile PCM cellsis especially fast (in comparison to flash or EEPROM devices) and thisproperty is important to interrupt the attack already at a very earlystage so that the damage can be limited.

Since further embodiments also comprise phase-change memory cells (orprogrammable resistive elements) comprising more than one irreversiblestate (e.g. a fourth state), it is possible that the different OTPstates (the irreversible states) can be used in order to code additionalinformation. It can for example be used to identify the circumstancesunder which the individual characteristics have been destroyed. It isalso possible to use an asymmetric phase-change memory cell comprisingmore than two reversible states, which are also known as multileveldevices.

FIG. 1 shows a schematic view of an electronic device comprising aprogrammable resistive element 110 and a circuitry 120, which areelectrically connected. The circuitry 120 is configured to provide aswitch signal to the programmable resistive element 110 and theprogrammable resistive element 110 comprises a first and a second state,and is configured to allow switching from the second state into thefirst state in response to a signal comprising at least a predefinedlevel. The predefined level can for instance comprise a predefinedvoltage, a predefined current, a predefined time, or a predefinedpolarity. The circuitry 120 is configured such that the switch signalcauses a switching from the first into the second state. The circuitry120 is further configured to provide signals only up to a maximum level(e.g. a maximal voltage V_(max) or a maximal current I_(max)), which isbelow the predefined level and, hence, the second state becomes anirreversible state, since the circuitry 120 is unable to provide asignal of the predefined level.

In other words, the circuitry 120 is configured such that a signalcomprising the predefined level may damage the circuitry 120 itself. Thepredefined level may comprise, e.g., a voltage which exceeds the workingvoltage of the circuitry 120, or when the predefined level comprises acurrent, which exceeds a maximum current allowable within the circuitry.The predefined level may also correspond to a specific polarization,which the circuitry is not adapted for.

Embodiments use for the programmable resistive element may comprisephase-change memory cell (comprising at least two states), asymmetricphase-change memory cell (comprising at least three states), CBRAM,metal oxide, etc. Phase-change memory cells are explained above andasymmetric phase-change memory cells will be described below in moredetail. CBRAM (conductive-bridging RAM) use, for example, an electrolytebetween two different metal electrodes in order to form ion-bridgesbetween the metal in one polarization, which disappear in the oppositepolarization. These ion-bridges change significantly the resistancebetween both electrodes, so that the high ohmic state corresponds to theabsence of ion-bridges, whereas in the low ohmic state ion bridges areformed. Therefore, in case of CBRAMs the signal of predefined levelneeds an opposite polarity than the switch signal and according toembodiments the circuitry is unable to provide the polarity related tothe predefined level (alternatively this polarity may damage or destroythe circuitry or its functionality). Said asymmetry can also be achievedby using materials comprising metal oxides as, for example, NiO.

FIGS. 2 a and 2 b show views of the asymmetric phase-change memory cell110 comprising a first electrode 111 and a second electrode 112connected by a phase-change material 113.

As the cross-sectional view of FIG. 2 a shows, between the first andsecond electrode 111 and 112, a first portion of a dielectric layer 115a is arranged and a second portion of the dielectric layer 115 b isarranged on the phase-change material 113, and on the first electrode111 and on the second electrode 112.

FIG. 2 b shows an overview of the asymmetric phase-change memory cell110 of FIG. 2 a, viewed from the side of the second portion of thedielectric layer 115 b, which is not shown in this FIG. 2 b. Only thefirst portion of the dielectric layer 115 a underneath the phase-changematerial 112 is visible. Again, the phase-change material 113, whoseasymmetric form is visible in this overview, bridges the first electrode111 and the second electrode 112. If a voltage is applied between thefirst and the second electrode 111,112 an electric current along acurrent path I may be generated and the asymmetric form of thephase-change material is such that the cross-sectional areaperpendicular to the current path changes along the current path. In theembodiments as shown in FIG. 2 b the phase-change material comprise aT-shape (defining a T-cell), wherein the cross section of the phasechange material 113 increases from the second electrode 112 to the firstelectrode 111.

As consequence, the phase-change material 113 comprises a T-shape inthat the phase-change material 113 comprises a small region 113 a (witha smaller cross-sectional area) and an enlarged region 113 b (with anincreased cross-sectional area). The first and second electrode 111, 112are separated by a distance L, which is bridged by the phase-changematerial 113 such that along the second portion of the dielectric layer113 b the small region 113 a extends over a first length La with a firstwidth Wa, and the enlarged region 113 b extends over a second length Lbwith a second width Wb. The T-shape implies that the first electrode 111is contacted over a smaller portion and that the second electrode iscontacted over an enlarged portion perpendicular to the current path I.

It is also possible to achieve the first and second cross-sectional areaby means of a funnel like shape in vertical direction.

The first length La may, for example, comprise a value between 100 nmand 800 nm, and the first width may comprise a minimal value between 10nm and 100 nm or between 20 nm and 80 nm.

FIGS. 3 a and 3 b show the different amorphous phases related to thesecond and third state, which are distinguished by the polarity at thefirst electrode 111 and the second electrode 112 (there may again adielectric layer 115 be arranged between the first electrode 111 and thesecond electrode 112). The different amorphous phases are related todifferent regions, in which the crystalline phase-change material 113becomes an amorphous region 116.

By applying a reset signal (e.g. the first or the second signal) thephase-change material 113 exhibits a phase-transition from a crystallinephase (with a small amount of resistance) to the amorphous phase. Theamorphous phase is however not formed along the whole phase-changematerial 113, but only within the amorphous region 116, whose positiondepends on the polarity between the first electrode 111 and the secondelectrode 112. This shift of this amorphous region 116 is due to theThomson Effect, which causes a shift in the temperature gradient in thepresence of an electric current along the phase-change material 113.This shift is towards the electrode with the +polarity and if thephase-change material 113 is asymmetric between the first and secondelectrodes 111, 112, this results into different (distinguishable)amorphous phases.

In addition to the shift of the position of the amorphous region 116also the volume of the amorphous region may differ. Since the amorphousphase exhibits a higher resistance the crystalline phase, an increasedvolume of the amorphous region 116 may yield an increased resistance.

FIG. 3 a shows an embodiment, wherein the amorphous region 116 is formedcloser to the second electrode 112 as an anode (+polarity) and definesthe third state (irreversible state). In comparison, in FIG. 3 b thepolarity is switched resulting to the amorphous region 116 being formedcloser to the first electrode 111 as anode, which defines the secondstate (reversible state).

In other embodiments the small region 113 a of the phase-change material113 may connect the first electrode 111 and the enlarged region 113 b ofthe phase-change material 113 may connect the second electrode. In thisembodiment, the polarity of FIG. 3 a would give the second state,whereas the polarity of FIG. 3 b would give the third state.

As explained above the temperature gradient fixes the position of theamorphous region 116 and is generated by the reset current passing theamorphous material 113. The three states realized in the phase-changematerial 113 are the crystalline phase with a low resistance and the twoamorphous phases related to shifts of the amorphous region 116 (cp.FIGS. 3 a and 3 b). The switch from the crystalline phase to theamorphous phases is the reset state and the switch from the amorphousphases to the crystalline phase is the set state.

In addition to the embodiment as shown in FIGS. 3 a and 3 b, in analternative embodiment the amorphous and crystalline phases areexchanged, i.e. the amorphous material 113 is in an amorphous phase andthe regions 116 in comprise the crystalline phase. In this embodimenttwo crystalline phases, related to different positions and/or volumes ofthe region 116, are combined with one amorphous phase, in which thewhole phase-change material 113 is amorphous. Also in this embodimentthe two crystalline phases comprise different resistances, e.g. causedby different volumes of the crystalline (low ohmic) region 116, which inthis case are both lower than the resistance of the amorphous phase.Hence, the irreversible state may in this embodiment the state with thelowest resistance or the one, which needs the biggest level signal toswitch into the amorphous state.

FIG. 4 shows the dependence of the reset resistance as a function of thereset current. These graphs may be obtained by taking medium values oversamples of T-cells, for example, with the first width Wa of 110 nm and afirst length La of 460 nm. Both graphs may be related to differentpolarities applied to the phase change memory cell.

FIG. 4 shows in detail two graphs, for example one of an N+ polarizationshowing the transition from the first state 410 to the third state 430,and the other for the N− polarization showing the transition from thefirst state 410 to the second state 420. The N+ polarization correspondsto the case as shown in FIG. 3 a, and the N− polarization corresponds tothe case as shown in FIG. 3 b. The phase transition between the firststate 410 and the second state 420 (with N− polarity) occurs at a firstcurrent I1, at which the reset resistance starts to grow rapidly. On theother hand, the phase transition between the first state 410 and thethird state 420 (with N+ polarity) occurs at a second current I2, atwhich again the reset resistance starts to grow rapidly.

Therefore, FIG. 4 shows three states, one state (the first state 410)with a low Ohmic resistance (e.g. a crystalline state) and two highOhmic states, the second state 420 and the third state 430. In addition,FIG. 4 shows that the third state 430 exceeds a resistance level Rc,which is such that a set is no longer possible (no return to thecrystalline phase). This impossibility to switch the third (amorphous)state 430 back to the crystalline state 410 is, e.g., due to the factthat a possible set signal would exceed a level (voltage or currentand/or polarity), which can be provided by the corresponding circuitry120.

FIG. 5 shows an embodiment for an electronic device with thephase-change memory cell 110 arranged within a dielectric layerstructure 115 of a MOS-device (MOS=Metal Oxide-Semiconductor). In FIG. 5the circuitry 120 (which may also comprise a configurable module) isformed in a substrate 130, which may for example comprise silicon anddoped regions within the silicon. The circuitry 120 comprises, forexample, a first part 121 which may comprise a doped region within thesemiconductor and a contact region 122. A dielectric layer structure 115comprising a first part 115 a and a second part 115 b is formed on thesubstrate 130. Within the dielectric layer structure 115 conductivelayer are arranged, wherein two of these conductive layers are used asthe first and the second electrode 111, 112 of the PCM-cell 110. Thefirst electrode 111 and the second electrode 112 are electricallyconnected by the phase material 113 comprising a length L0.

The contact region 122 is contacted by a via contact 123 providing anelectric connection between the contact region 122 and the secondelectrode 112, and which is arranged between the first part and secondpart 115 a and 115 b of the dielectric layer structure 115. In addition,the first electrode 111 is also arranged between the first and secondpart of the dielectric layers 115 a and 115 b. The phase-change material113 may comprise a symmetric or the asymmetric shape as shown in FIG. 2b, when viewed from the topside (opposite to the substrate 130).

Arranging the asymmetric phase-change memory cell 110 within thedielectric layer structure 115 of the MOS-device provides thepossibility of hiding the functionality of the phase-change memory cell110. This comprises a major advantage, which is especially important forsecurity-related devices, wherein concrete connections and switchingbehavior between different devices should remain hidden.

In further embodiments the phase-change memory cell 110 is replaced byan OTS-cell, which optionally may be part of a logic circuit realized inpart in the substrate 130.

Embodiments comprise also a method for manufacturing an MOS device,comprising a forming of the asymmetric phase-change memory cell formedwithin the dielectric layer structure and a connecting the asymmetricphase-change memory cell a semiconductor device in a substrate(comprising for example silicon).

Further embodiments comprise a method for blocking an electronic devicecomprising a programmable resistive element (or an asymmetric phasechange memory cell) and a circuitry, wherein the programmable resistiveelement (or an asymmetric phase change memory cell) comprises a first, asecond and a third state. The method comprises a switching between thefirst, the second, and the third state, wherein switching from the thirdstate into the first or into the second state is performed in responseto a signal comprising at least a predefined level. The method furthercomprising a providing a first and a second signal by the circuitry,wherein the first signal causes switching between the first and thesecond state and the second signal causes switching from the first orfrom the second state into the third state, wherein the circuitry isconfigured to provide signals up to a maximum level, which is below thepredefined level.

1. An electronic device comprising: a programmable resistive elementcomprising at least a first state and a second state, wherein theprogrammable resistive element is configured to allow switching from thesecond state into the first state in response to a signal comprising atleast a predefined level; and a circuitry for providing signals belowthe predefined level, wherein the circuitry is configured to provide aswitch signal to the programmable resistive element, wherein the switchsignal causes switching from the first into the second state.
 2. Theelectronic device of claim 1, wherein the programmable resistive elementis configured such that the predefined level comprises a predefinedvoltage or a predefined current or a predefined polarity or a predefinedtime, and wherein the circuitry is unable to provide the predefinedvoltage or the predefined current or the predefined polarity.
 3. Theelectronic device of claim 1, wherein the circuitry comprises a detectorfor detecting an unauthorized access of the circuitry and theprogrammable resistive element is configured to switch into the secondstate in case the detector detects the unauthorized access.
 4. Theelectronic device of claim 1, wherein the programmable resistive elementfurther comprises a third state and a memory material such that thesecond and the third state comprise both high ohmic states of the memorymaterial with different resistances.
 5. The electronic device of claim4, wherein the circuitry is configured to provide signals for switchingbetween the first and the third state.
 6. The electronic device of claim5, wherein the predefined level comprises a predefined voltage, andwherein the programmable resistive element comprises a fourth statecomprising a further state of the memory material and wherein theprogrammable resistive element is configured to allow switching from thefourth state into the first or into the third state in response to asignal comprising at least the predefined level.
 7. The electronicdevice of claim 1, wherein the programmable resistive element comprisesa phase change memory element comprising a phase change material.
 8. Theelectronic device of claim 7, wherein the programmable resistive elementfurther comprises a third state such that at least the second statecomprises a mixture of an amorphous phase and a first crystalline phaseand the third state comprises a mixture of an amorphous phase and atleast a second crystalline phase with different resistance than thefirst state, or at least the second state comprises crystalline phasesof the phase change material with different resistance than the firststate.
 9. The electronic device of claim 1, wherein the programmableresistive element comprises a T-cell.
 10. The electronic device of claim1, wherein the programmable resistive element comprises a funnel likeshape.
 11. The electronic device of claim 1, wherein the circuitry isconfigured to provide only signals below the predefined level.
 12. Theelectronic device of claim 7, wherein the phase change materialcomprises a chalcogenide or a chalcogenide alloy.
 13. The electronicdevice of claim 1, wherein the programmable resistive element comprisesa metal oxide.
 14. An electronic device comprising: a programmableresistive element comprising at least a first state and a second state;and a circuitry for providing a switch signal to switch the programmableresistive element from the first state into the second state, whereinthe circuitry is unable to provide a signal to switch the programmableresistive element from the second state into the first state.
 15. Theelectronic device of claim 14, wherein the programmable resistiveelement further comprises a third state, the first state and the thirdstate are reversible states in that the circuitry is able to providesignals to switch between the first state and the third state.
 16. Theelectronic device of claim 15, wherein the programmable resistiveelement comprises a memory material such that the first state, thesecond state and the third state comprise different resistances, whereinthe resistance of the third state is between the resistance of the firststate and the resistance of the second state, and wherein the secondstate comprise a highest resistance.
 17. The electronic device of claim16, wherein the memory material is a phase change material, wherein thefirst state comprises a crystalline phase, and wherein both, the secondstate and the third state comprise different amorphous phases ordifferent mixtures of crystalline phases with amorphous phases.
 18. Theelectronic device of claim 15, wherein the programmable resistiveelement further comprises a fourth state, and wherein the circuitry isconfigured to provide a further switch signal to switch from the firststate into the fourth state, wherein the fourth state is irreversible inthat the circuitry is unable to provide a signal to switch from thefourth state into the first state or into the third state.
 19. Theelectronic device of claim 18, wherein the programmable resistiveelement comprises a memory material such that the first state, thesecond state, the third state, and the fourth state comprise differentresistances, wherein the fourth state comprise a lowest resistance andthe second state a highest resistance, and wherein the first statecomprise a lower resistance than the third state.
 20. The electronicdevice of claim 19, wherein the memory material is a phase changematerial, wherein both, the first state and the fourth state comprisedifferent crystalline phases, and wherein both, the second state and thethird state comprise different amorphous phases.
 21. The electronicdevice of claim 15, wherein the programmable resistive element comprisesan asymmetric phase change memory cell.
 22. An electronic devicecomprising: a programmable resistive element comprising a phase changematerial switchable from a first or second crystalline into an amorphousphase, wherein the first and second crystalline phases having differentresistances of the phase change material, and wherein the programmableresistive element is configured to allow switching from the secondcrystalline phase into the amorphous or into the first crystalline phasein response to a signal comprising at least a predefined voltage; and acircuitry configured to provide a first and a second signal to theprogrammable resistive element, wherein the first signal causesswitching between the first crystalline and the amorphous phase, and thesecond signal causes switching from the amorphous or from the firstcrystalline phase into the second crystalline phase, and wherein thecircuitry is configured to provide signals below to a maximal voltage,which is below the predefined voltage.
 23. The electronic device ofclaim 22, wherein the programmable resistive element comprises a firstand a second electrode electrically connected by the phase changematerial along an electric current path, wherein a cross sectional areaperpendicular to the current path increases from the first electrodetowards the second electrode.
 24. The electronic device of claim 23,wherein the first and the second electrode are connected by the phasechange material along a length, which is between 100 nm and 800 nm, andwherein the cross section area comprises a minimal width, which isbetween 10 nm and 100 nm.
 25. An electronic device comprising: anasymmetric phase change element comprising at least a first, a secondand a third state, wherein the phase change element is configured toallow switching from the at least third state into the first or into thesecond state in response to a signal comprising at least a predefinedlevel; and a circuitry configured to provide a first and a second signalto the asymmetric phase change element, wherein the first signal causesswitching between the first and the second state and the second signalcauses switching from the first or from the second state into the thirdstate, and wherein the circuitry is configured to provide signals up toa maximum level, which is below the predefined level.
 26. The electronicdevice of claim 25, wherein the circuitry is configured to provide adevice individual bit stream, and wherein the circuitry is configured toblock providing the bit stream in case the asymmetric phase changememory element is within the third state.
 27. The electronic device ofclaim 26, wherein the circuitry comprises an integrated circuitrealizing a physically uncloneable function for providing the devicespecific bit stream.
 28. The electronic device of claim 25, wherein thecircuitry is configured such that the circuitry is damaged in case asignal of the predefined level is applied.
 29. The electronic device ofclaim 25, further comprising a detector for detecting an unauthorizedattempt to access the circuitry and wherein the asymmetric phase changeelement is configured to switch into the third state in case thedetector detects the unauthorized attempt.
 30. An apparatus for blockingan electronic device comprising: a means for switching between at leasta first, a second, and a third state, wherein the means for switching isconfigured to switch from the at least third state into the first orinto the second state in response to a signal comprising at least apredefined level; and a means for processing configured to provide afirst and a second signal to the means for switching, wherein the firstsignal causes switching between the first and the second state, and thesecond signal causes switching from the first or from the second stateinto the at least third state, and wherein the means for processing isconfigured to provide signals up to a maximum level, which is below thepredefined level.
 31. The apparatus of claim 30, further comprising ameans for detecting an attempt of unauthorized access of the means forprocessing, wherein the means for switching is configured to switch incase the attempt is detected from the first or from the second stateinto the at least third state.
 32. A method for blocking an electronicdevice comprising an asymmetric programmable resistive element and acircuitry, wherein the asymmetric programmable resistive elementcomprises a first, a second and at least a third state, comprising:switching between the first, the second, and the at least third state,wherein switching from the third state into the first or into the secondstate is performed in response to a signal comprising at least apredefined level; and providing a first and a second signal by thecircuitry, wherein the first signal causes switching between the firstand the second state and the second signal causes switching from thefirst or from the second state into the third state, wherein thecircuitry is configured to provide signals up to a maximum level, whichis below the predefined level.
 33. The method of claim 32, wherein theprogrammable resistive element is an asymmetric phase change memoryelement.
 34. The method of claim 32, further comprising detecting anattempt of unauthorized access of the circuitry and a switching from thefirst or from the second state into the third state.